This invention relates to an improved process for the bromination of diphenylalkanes.
Brominated diphenylalkanes, e.g., decabromodiphenylethane, are known flame retardants for use in polystyrene and polyolefin-based thermoplastic formulations. It is predicted that decabromodiphenylethane will soon become one of the major flame retardants used by the thermoplastic industry. In response to this market opportunity, several decabromodiphenylethane processes have been proposed. See U.S. Pat. No. 5,077,334, U.S. Pat. No. 5,008,477 and U.S. Pat. No. 5,030,778.
While these processes are quite efficacious, there is always a desire to develop more economical and technologically beneficent processes. It is an object of this invention to provide such a process.
This invention provides a unique process for producing an intermediate decabromodiphenylethane slurry from which a decabromodiphenylethane wet cake can be more efficiently obtained. Even further, the obtained wet cake is most easily convertible to a high-quality ready-to-use flame-retardant product. The wet cake is characterized by having a lower occluded bromine content than that which is obtainable by prior processes.
The process of this invention comprises: mixing bromine and diphenylethane, the molar ratio of bromine to diphenylethane being greater than about 5:1, but preferably less than about 30:1; and quickly feeding the resultant mix to a stirrable reaction mass comprising bromine and a bromination catalyst to yield decabromodiphenylethane.
It is theorized, though this invention is not to be limited to a particular theory, that the feeding of the bromine/diphenylethane derived mixture (which mixture is very dilute in diphenylethane and/or underbrominated diphenylethane) to the reaction mass favorably affects the crystallization of the decabromodiphenylethane product in the reaction mass so that there is a reduction in the formation of extremely small particles (fines) and so that there is an attenuation of the occluded free bromine content in the crystalline structure. It is believed that by having the bromine present in diluent quantities, as the derived mixture is fed, there is obtained, in the area of the feed, (1) a minimization of the variability of the brominated diphenylethane concentration, and (2) the unlikelihood that the crystallization medium, i.e., the reaction mass, will become excessively supersaturated with decabromodiphenylethane. Thus, good, slowed crystal growth is promoted and crystal nucleation is abated.
In addition, the diluent function of the fed bromine benefits product color. It is theorized that when the derived mixture is fed to the reaction mass there is a transient feed plume formed in the reaction mass. There, the diluent bromine acts as a mass transfer impediment to impede the bromination catalyst contained in the reaction mass from reaching some of the yet to be brominated or underbrominated diphenylethane located in the plume. This is beneficial since it is believed that the bromination catalyst will attack, i.e., cleave, the diphenylethane xe2x80x94Cxe2x80x94Cxe2x80x94 bridge if there is insufficient or no bromination of the diphenylethane prior to contact with the catalyst. The cleaved materials are, in many instances, undesirable color bodies. By impeding the mass transfer of the catalyst, for even a very short period of time, more diphenylethane will have sufficient time to obtain the degree of bromination needed to deter cleavage. Once the plume is dissipated in the reaction mass, and this occurs quickly, the bromination catalyst can then effectively catalyze the reaction to obtain the desired ar-brominated decabromodiphenylethane product.
In addition to diminishing xe2x80x94Cxe2x80x94Cxe2x80x94 cleavage, the dilution effect favorably affects the formation of color bodies by reducing the concentration of bromination loci per unit volume. Since the bromination loci are exothermic, the reduction in their concentration enables the avoidance of obtaining color body producing degradation temperatures at each individual locus. The large amount of bromine around each reaction locus acts as heat sink so that the heat is effectively dissipated.
Generally speaking, the processes of this invention will be most useful to the commercial decabromodiphenylethane producer who deals with large reaction volumes, say larger than about 1,000 L (250 gal). Most commercial reactions will be sized from this minimum up to about 32,000 L (8,000 gal). It is in dealing with large reaction masses where the problems associated with decabromodiphenylethane production are most easily seen as mass transfer and crystallization quality are more problematic.